Fast and facile synthesis of silica coated silver nanoparticles by microwave irradiation

Abstract

A novel, fast and facile microwave technique has been developed for preparing monodispersed silica coated silver (Ag@SiO₂) nanoparticles. Without using any other surface coupling agents such as 3-aminopropyltrimethoxysilane (APS) or polymer such as polyvinyl pyrrolidone (PVP), Ag@SiO₂ nanoparticles could be easily prepared by microwave irradiation of a mixture of colloidal silver nanoparticles, tetraethoxysilane (TEOS) and catalyst for only 2 min. The thickness of silica shell could be conveniently controlled in the range of few nanometers (nm) to 80 nm by changing the concentration of TEOS. Transmission electron microscopy (TEM) and UV–visible spectroscopy were employed to characterize the morphology and optical properties of the prepared Ag@SiO₂ nanoparticles, respectively. The prepared Ag@SiO₂ nanoparticles exhibited a change in surface plasmon absorption depending on the silica thickness. Compared to the conventional techniques based on Stöber method, which need 4–24 h for silica coating of Ag nanoparticles, this new technique is capable of synthesizing monodispersed, uniform and single core containing Ag@SiO₂ nanoparticles within very short reaction time. In addition, straightforward surface functionalization of the prepared Ag@SiO₂ nanoparticles with desired functional groups was performed to make the particles useful for many applications. The components of surface functionalized nanoparticles were examined by Fourier transform infrared (FT-IR) spectroscopy, zeta potential measurements and X-ray photoelectron spectroscopy (XPS).

Introduction

Metallic nanoparticles have attracted considerable attention both fundamentally and technologically because of their unique electric and optical properties that are not observable in their bulk counterparts [1], [2]. These nanostructured materials have been receiving increased attention in the past decades owing to their applications in the field of catalysis, optics and biosensing [3], [4], [5], [6], [7], [8]. The unique optical properties of metal nanoparticles arise from collective oscillation of conduction electrons upon interaction with electromagnetic radiation, the so-called localized surface plasmon resonance (LSPR) [9]. The LSPR extinction spectrum is highly sensitive to the size, shape and local refractive index (RI) near the surface of the metal nanoparticles as well as their interparticle distances, which is the basis of using LSPR as molecular sensing [10], [11]. Particularly, nanostructures made from the noble metals, such as those of silver (Ag) and gold, with their associated strong LSPR, have generated great interest in recent years [12], [13], [14]. These noble metal nanoparticles show absorption band in the visible region (380–750 nm) by their LSPR. However, an increase in RI of the surrounding environment of these metal particles leads to a red shift of the LSPR band [15]. Therefore, adsorbate-induced local refractive index change (due to some specific molecular recognition or adsorption) around the surface of the noble nanoparticles induces shifting of LSPR which can be monitored easily by scattering or adsorption technique using a simple and cost effective optical experimental setup [10]. Based on these specific optical characteristics of LSPR and the experimental features of noble metal nanoparticles, nanosensors were designed for the detection of biological targets, binding kinetics, and protein conformational changes [10].

Potentially the best metal in these purposes is Ag as it is inexpensive relative to other materials, possesses good chemical and physical properties [8], [16], [17] and has high molar extinction coefficient (100× greater than that of gold). In addition, Ag nanoparticles provide significant enhancements of Raman scattering (greater than gold) [18]. Therefore, this nanoparticle also has potential application as optical sensors using surface enhanced Raman scattering. However, the successful integration of Ag nanoparticles into useful devices is insignificant. This is due to the fact that Ag is sensitive to the surrounding chemicals, causing easy oxidization and significant degradation of its plasmonic properties. Moreover, in solution, Ag nanoparticles aggregate due to van der Waals forces [19]. In order to overcome these problems, Ag nanoparticles often require application-specific coatings to passivate and/or fuse the nanoparticle to target molecules or surfaces [20]. For maximum versatility, the ideal coating should have multifunctional ability, simple way to conjugate with the target molecules and the ability to adjust its morphology to control the degree to which the encapsulated nanoparticle interacts with its surrounding environment. Silica is an excellent candidate for such a generalized platform coating as it is chemically inert in a wide variety of solvents, highly transparent in the visible and IR regions of the spectrum and can easily be functionalized with silane coupling agents for further bioconjugation [21], [22].

Recently, a number of investigations have been published on the direct silica coating of colloidal particles of metal [21], [22], [23], [24], [25], [26], metal oxide [27], [28], [29], [30], [31], [32], [33], [34] and semiconductors [35] based on Stöber method [36]. This process typically involves ammonia-catalyzed hydrolysis and condensation of molecules, such as tetraethoxysilane (TEOS). However, direct application of this method to coat Ag nanoparticles possesses many difficulties such as low chemical affinity of Ag to silica, rapid oxidation and aggregation of Ag [23]. Mulvaney and coworkers [19] synthesized silica coated Ag nanoparticles using a silane coupling agent (e.g. 3-aminopropyltrimethoxysilane, APS). Although the process reported by Mulvaney is useful for industrial applications, it has many disadvantages, such as, multistep procedure with long reaction duration and requires additional pre-coating step by toxic coupling agent (APS) that limit bio-applications. Instead of silane coupling agent, Graf et al. [37] used polyvinylpyrrolidone (PVP) as primer and modified Ag nanoparticles. The method was faster but the homogeneity and smoothness of silica coating was influenced by the length of polymer. Kobayashi et al. [23] developed a more effective method to coat the Ag nanoparticles without silane coupling agent. However, the reported method needed long time and limited to coat the Ag nanoparticles with very thin silica layer (5–10 nm). Han et al. [38] reported reverse microemulsion method to coat Au and Ag nanoparticles. This method also required long reaction time (5 h) and an additional reagent polyoxyethylene(5)nonylphenyl ether. Moreover, thickness of silica around Ag nanoparticles was not uniform. In comparison to these well established approaches, it is, therefore, necessary to develop more efficient methods of synthesis involving simplified preparation procedure with short reaction duration. The use of microwave (MW)-assisted synthesis is especially important in this regard.

The use of MW irradiation instead of conventional heating in various chemical reactions has been receiving increased interest by many research groups in the past decade [39], [40]. As Compared to the conventional techniques, some of the controlled microwave heating processes showed a dramatic reduction of reaction times, increased production yields, reduction in the production cost, decrease of environmental pollution and a higher purity of the products as a result of reducing unwanted side reactions [40], [41]. The exact nature of microwave interaction with reactants during the synthesis of materials is somewhat unclear and speculative [39]. However, energy transfer from the microwaves to the materials is achieved through the interaction of microwave radiation with water or other solvents of high dielectric constant or solvents molecules with large dipole moments [42]. Recently, we have demonstrated the synthesis of silica coated ZnO, TiO₂, CeO₂ and TiO₂ coated SiO₂, ZnO nanoparticles by the MW technique for the first time [29], [30], [31], [32], [33], [34]. These reports have shown that under microwave radiation the reaction time is extraordinarily short and the coating layers are more uniform compared to conventional methods. However, to our best knowledge, silica coating of colloidal metals by MW method has not been attempted yet. In this paper, MW irradiation is applied to synthesize silica coated Ag nanoparticles (Ag@SiO₂) without an intermediate coating step. MW method enables to adjust the silica thickness with a wide range of 5–80 nm. Additionally, the present method facilitates to synthesize monodispersed Ag@SiO₂ nanoparticles with uniform silica shell around Ag. The prepared Ag@SiO₂ nanoparticles have also been functionalized with specific groups for further bioconjugation.